Titanium oxide has a very wide field of industrial application, and is versatile which can be used typically as an additive to cosmetics, ultraviolet screening materials and silicone rubber, and in recent years, in a photocatalyst, solar cells, a material of dielectric bodies and a material of an electrode for Li ion batteries as well. Although it is described as “titanium dioxide” in Japanese Industrial Standards (JIS), it is to be abbreviated as “titanium oxide” in the present description because “titanium oxide” is widely used as a general name.
Recently ultrafine particulate titanium oxide is drawing attention as a material for a high-performance dielectric body and an electrode of Li ion batteries. The range of the diameter of the primary particles of ultrafine particulate titanium oxide is not clearly defined in general. While the term “ultrafine particles” is generally used for fine particles having a diameter of about 100 nm or less, the ultrafine particulate titanium oxide of the present invention is titanium oxide having an average primary particle diameter (DBET) of 3 to 8 nm calculated on the basis of the BET specific surface area as described below.
For example, Li4Ti5O12 as a typical material of the electrode for Li ion batteries is generally obtained by a solid phase reaction between a lithium material and titanium oxide. Specifically, Li4Ti5O12 is produced by the process of uniformly mixing a lithium material and titanium oxide, the process of drying the mixture and the process of treating the mixture with heat. Lithium hydroxide, lithium oxide, lithium carbonate and the like can be used as a lithium material. Titanium oxide is mixed into water in which a lithium material is dispersed. The titanium oxide to be used is preferably anatase-type titanium oxide or aqueous lithium oxide, which has higher reactivity than rutile-type titanium oxide.
It is critical that the titanium has high dispersivity to fulfill its functions. Since variation in the reactivity and quality of the titanium oxide is determined depending on the mixing state in the above-mentioned solid phase reaction, titanium oxide having a low agglomeration degree and high dispersivity is required. If titanium oxide having low dispersivity is used, the process of deflocculating aggregates is needed, which may require excessive energy for crushing aggregates or cause problems of contamination of an abrasive substance or uneven particle size.
High dispersivity is also required for titanium oxide when it is used as a photocatalyst. Since low dispersivity leads to enhancing opacifying property, it will limit the usefulness of the titanium oxide. In the field of solar cells, titanium oxide having low dispersivity is hard to let light through and therefore the titanium oxide contributing to light absorption is to be limited, reducing the incident photon-to-current (conversion) efficiency.
The methods for producing titanium oxide are to be classified broadly into two kinds: i.e. a liquid phase method of hydrolyzing titanium tetrachloride or titanyl sulfate and a gas phase method of reacting titanium tetrachloride with an oxidation gas such as oxygen and water vapor. Titanium oxide powder having high crystalinity and excellent dispersivity can be obtained by the gas phase method, however, since it is obtained by reaction at a temperature higher than 500° C., particle growth and sintering of particles to each other proceed during the reaction and titanium oxide having a specific surface area of 200 m2/g or more cannot be obtained efficiently (JP-A-2006-265094 (WO 2006/098175 publication); Patent Document 1). Since titanium oxide obtained by the liquid phase method is generated at a temperature from ordinary temperature to about 300° C. at the highest, particle growth is suppressed and ultrafine titanium oxide particles are easily obtainable.
As a method of obtaining titanium oxide having high dispersivity by the liquid phase method, a method of modifying the surface of titanium oxide with a dispersing agent such as silica, alumina and an organic compound in an effort to maintain the dispersibility of slurry or sol over a long time period has been reported. For example, JP-A-2004-043304 (Patent Document 2) discloses a method of dispersing and stabilizing titanium oxide using at least one member of acids selected from hydroxycarboxylic acids.
Since impurities are to be added to titanium oxide in each of the methods as taught by the above-mentioned patent documents, it may not be suitable depending on the uses. When titanium oxide is used for dielectric materials, solar cells or photocatalysts, if an ingredient having corrosivity such as chlorine exists, it may lead to corrosion or deterioration of the substrate, it is necessary to keep the chlorine content in the titanium oxide as low as possible. Also, it is better to keep the contents of iron (Fe), aluminum (Al), silicon (Si), sulfur (S) and the like as low as possible. Impurities should be rejected as much as possible for the use in a dielectric material and an electrode material since the impurities adversely affect the electrical properties of the materials. When titanium oxide is for use in photocatalysts and solar cells, titanium oxide containing Fe is not suitable for use where transparency is required since Fe in the titanium oxide causes coloring; and titanium oxide having a high content of Al, S and the like causes lattice defect and degrades the performance of the materials.
As discussed above, it has been difficult to obtain anatase-type titanium oxide having high dispersivity and high purity according to the conventional methods.